The present invention relates to a method of position sensing and more particularly to a method for sensing steering wheel position.
It is well known in the art that the voltage modulation of Hall elements or resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a tooth, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth is adjacent to the MR than when a tooth is at a distance from the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
The crank position information is encoded on a rotating target wheel in the form of teeth and slots. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g. Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of greater encoding flexibility and speed independent output signals.
High accuracy and repeatability magnetic position sensors employ two matched sensing elements such as magnetoresistors (MR) or Hall generators. They are spaced a few mm apart from each other. The primary purpose of using two matched sensing elements is common mode signal rejection, since the sensing elements are equally affected by temperature and air gap.
In addition, Electric Power Steering (EPS) controls need to know the absolute position of the steering wheel at all times, including the instant of powering on the system. The steering wheel can be rotated three full revolutions in each direction. However, the specifications call for the sensor itself to have a minimum capability of several full rotations in each direction. Present sensors are incapable of recognizing multiple revolutions, and must be augmented by software. The position learning algorithm requires time to figure out the steering wheel position, and it is incapable of recognizing position at power on. A simple and inexpensive non-contact absolute steering wheel position sensor, even with only a moderate resolution, would be highly desirable.
What is needed is a method and apparatus wherein a simple and inexpensive non-contact sensor, even with only a moderate resolution, gives absolute steering wheel position at all times.
The present invention provides a method and apparatus wherein a single dual element galvanomagnetic sensor, herein exemplified by a single dual element magnetoresistive (MR) sensor, is incorporated to sense absolute steering wheel position from the position of a magnetic irregularity, such as for example a ferromagnetic tooth or a slot in a ferromagnetic material, with respect to the stationary MR sensor.
Such a steering wheel sensor can be constructed by threading a short section of the steering shaft and placing a threaded article, for example a nut with a matching thread, into threaded engagement with the threaded section. The nut is prevented from rotating by means of a sliding bar attached to the nut which is received within a guiding channel of a stationary MR sensor assembly such that the nut is enabled to move axially with respect to the stationary MR sensor. The sliding bar is fitted with a ferromagnetic tooth or the sliding bar is, itself, ferromagnetic and has a slot. The axial direction of movement is such that it is along a line joining the centers of the two MR elements of the single dual element MR sensor (i.e. MR sensor).
Thus, the rotation of the steering wheel is translated into a precisely repeatable axial movement of the nut, and, as a consequence, a precisely repeatable axial movement of the tooth or slot with respect to the stationary MR sensor. And, since the resistance of the MR elements of the MR sensor are responsive to magnetic field intensity changes, the output of the MR sensor is related to the axial position of the nut with respect to the MR sensor.
For example, one full revolution of the steering wheel could vary the axial position of the nut by one thread pitch. The edge of the sliding bar has a slot or tooth which is sensed by the MR. The output of the MR sensor, with proper circuit design, indicates the position of the tooth (or the slot) with respect to the stationary MR sensor, and, hence, data pertaining to the net rotation of the steering wheel, be that clockwise (CW) or counterclockwise (CCW). If desired, this MR sensor could be packaged as one sensing system together with a torque sensor.
For simplicity of installation, the nut could be designed in a way which would permit mounting it by transverse placement onto the steering shaft without the necessity of a free end. One possible approach is the use of a nut consisting of two separate halves. These halves can be easily connected by means of clips. Alternatively, the nut could be manufactured from a resilient material (e.g. spring steel or polymer) with a cut-out to permit the threaded section of the steering shaft to be forced through the cut-out via a resilient (and temporary) deformation of the nut and thereby allow placement onto the threaded section.
Accordingly, it is an object of the present invention to provide a method and apparatus wherein a single dual element galvanomagnetic sensor, herein exemplified by a single dual element magnetoresistive sensor, is incorporated to sense absolute steering wheel position from the position of a tooth or a slot with respect to a galvanomagnetic sensor.
This, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification.